1. Field of the Invention
The present invention relates to a data recording/reproducing device and, more particularly, to a track access device for accessing a desired track on a recording medium, such as disc, having a large number of tracks, and also to a tracking control device.
2. Description of the Prior Art
An optical recording and reproducing device is known which utilizes a recording medium having a number of tracks formed thereon.
The optical recording and reproducing device is arranged to concentrate light beams generated from a light source using a condensing lens and to irradiate the same onto an information recording medium having tracks so as to record and reproduce signals. The information recording medium, such as a disk, is made by forming a film of material which is capable of optically recording and reproducing on a substrate surface concentric circular tracks of uneven structure using a vapor deposition method, etc. As the light source, for example, a semiconductor laser diode is used. The reproduction of signals is performed by setting the light beam at a relatively weak predetermined light intensity and by reading out the intensity of the reflected light from the disk. The recording of signals is conducted by modulating the light beam intensity in accordance with each signal to be recorded.
In such an optical recording and reproducing device, a focusing control is provided for controlling the concentration of light beams onto the recording material film approximately maintain a predetermined focused state at all times. Furthermore, a tracking control is provided so that the light beams are incident on a predetermined track at all times.
In the optical recording and reproducing device, in order to allow the light beam to randomly access a track on the disk, a track access control is effected. The track access control is effected by making the tracking control non-operative, moving the light beam radially on the disk towards the target track, and again actuating the tracking control when the light beam has reached the target track. The preceding technology regarding track access is disclosed, for example, in U.S. Pat. No. 4,106,058 or No. 4,332,022, etc.
One of the important factors in the track access control is the velocity of the light beam when it crosses the tracks, that is, the tracking pull-in velocity. The control band of the tracking control is limited, and normally in the range of several KHz. Therefore, when the tracking pull-in velocity is too fast, the tracking control into the target track fails. On the contrary, when the tracking pull-in velocity is too slow, it takes time to access the target track.
Therefore, the velocity control for controlling the velocity of the light beam is effected when moving the light beam radially on the disk in the track accessing process. The object of the velocity control is to accurately control the track pull-in velocity so as to achieve a stable pull in of the tracking control into the target track.
The track access is effected by moving the light beam radially on the disk so that the pull-in velocity of the light beam is set to a predetermined reference velocity determined corresponding to the current position of the light beam along the radial direction of the disk.
The light beam moving velocity necessary to effect the velocity control is detected in accordance with the period of the track crossing signal generated when the light beam crosses tracks. For the track access operation, the current position of the light beam is obtained by counting the number of track crossing signals from the starting track from which the track access operation starts.
In FIG. 2, the tracking error signal and the track crossing signal generated when the light beam crosses tracks in the radial direction of the disk are shown. Particularly, FIG. 2(a), shows the state where the light beam crosses tracks on the disk and FIG. 2(b) shows the tracking error signal. On the disk, pregrooves of uneven structure having a optical depth of approximately .lambda./8 (.lambda. is a wavelength of the light beam) are formed at a predetermined pitch. A band between adjacent pregrooves is a track where information is recorded.
A disk of such a structure is called an on-land disk. In an on-land disk, the tracking error signal can be generated by a push-pull method as shown in FIGS. 2(a) and 2(b). Since the push-pull method is described in detail, for example, in Japanese Patent Publication No. 59-9085 issued Feb. 29, 1984 or in French Patent No. 7529707 issued Sep. 29, 1975, the explanation thereof is omitted.
FIG. 2(c) depicts is a digital signal obtained by digitalizing the tracking error signal with the use of a hysteresis circuit, and FIG. 2(c) depicts the edge detection signal obtained by detecting the leading edge of the digital signal. Each pulse in the edge detection signal occurs when the light beam crosses the center of each track, and therefore, it represents the track crossing signal. Therefore, the edge detection signal shown in FIG. 2(d) is hereinafter referred to as a track crossing signal. Therefore, the count obtained by counting the track crossing signals from the starting of the track access operation represents the current position of the light beam. Such a position detection during the track accessing according to this method is referred to as the groove count method.
Also, since tracks are arranged approximately at the same pitch P in the radial direction of the disk, the light beam radial velocity V is obtained by EQU V=P/T,
in which T is a period of one cycle of the track crossing signal.
The radial velocity detection according to this method is referred to as the period measurement type detection method.
In FIG. 2(e), pulses having a period t are shown which represent leading and trailing edges of the pulse shown in FIG. 2(c). Thus, the radial velocity of the light beam can be given as EQU V=P/2t.
There are two methods for writing information onto the disk: one is to change the intensity of the light beam in accordance with the information to be recorded; and the other is to form pits during the manufacture of the disk along the tracks, as realized in compact disks. The latter method has an advantage in that many copies can be made at a low cost by the mastering replication technology used in the compact disk, and is suitable for disk storing of software such as encyclopedias, dictionaries, atlases, and operation systems.
In FIG. 3(a), a disk is shown which has an outer recording area RD for recording data by modulating the intensity of the light beam in accordance with to be recorded information, and an inner ROM area RM in which information is recorded in advance by the formation of a number of pits. Also, in FIG. 3(b), a fragmentary enlarged view is depicted, particularly showing the boundary portion between the recording area RD and the ROM area RM. As is apparent from FIG. 3(b), the tracks in the recording area RD have a relatively even surface, while the tracks in the recording area RM have an uneven surface.
In FIG. 4, signals are shown which are generated when the light beam crosses the tracks in the radial direction of the disk in the recording area RD and also in the ROM area RM. Specifically, FIG. 4(a) shows the tracking error signal, and FIG. 4(b) shows the binary signal obtained by digitalizing the tracking error signal using a hysteresis circuit. FIG. 4(c) shows the track crossing signal obtained by detecting the leading edges of the binary signal, and FIG. 4(d) shows pulses obtained by detecting the leading and trailing edges of the binary signal. In FIG. 4, the abscissa represents time and narrow pulses are shown by arrows.
When the period S.sub.2 of the sinusoidal noise signal imposed on the tracking error signal in ROM area RM is compared with the period S.sub.1 of the tracking error signal in the recording area RD, a relationship EQU S.sub.1 &gt;&gt;S.sub.2
is obtained. The short period S2 of the noise signal in the ROM area is caused by the deterioration of the S/N ratio in the tracking error signal which is erroneously modulated by the presence of pits. The amplitude of the tracking error signal in the ROM area is decreased due to the presence of the pits, and an offset amount V.sub.2 is produced due to the non-uniformity of the pit shapes. Accordingly, at time regions K.sub.1 and K.sub.2, a pulsating noise signal having an substantially shortened period is generated when the track error signal is changed to the binary signal.
Furthermore, in the time region K.sub.3, since the amplitude of the signal is substantially reduced, the hysteresis circuit fails to detect the low amplitude tracking error signal, and therefore, no pulse will be present in the binary signal in the region K3. Thus, the pulse spacing period is lengthened.
Since the radial velocity of the light beam is detected from the period of the track crossing signal, an erroneous detection of the light beam radial velocity takes place. As a result, an abnormal velocity variation takes place and the track pull in of the tracking control may fail.
Furthermore, the position of the light beam is erroneously detected, and the track detection operation may be terminated at a position other than the target track. As a result, it becomes necessary to again conduct the track access operation in order to correctly reach the target track, resulting in a drawback in that the track access time becomes longer.
In order to further shorten the time required for the track access, it is necessary to move the light beam at a high radial velocity. In FIG. 5 depicts the tracking error signal and the track crossing signal generated by the prior art track access device when the light beam is moved at a high radial velocity so that the period S.sub.1 of the tracking error signal becomes nearly equal to the period S.sub.2 of the signal recorded in the ROM area. In FIG. 5(a) shows the tracking error signal at such a condition, FIG. 5(b) shows the binary signal obtained by digitalizing the tracking error signal using a hysteresis circuit, and FIG. 5(c) shows the track crossing signal obtained by detecting the rising edges of the binary signal. Furthermore, FIG. 5(d) shows the signal obtained by detecting the rising and falling edges of the signal of FIG. 5(b). The period S.sub.2 of the signal recorded in the ROM area and the period S.sub.1 of the tracking error signal become nearly equal to each other. As a result, the tracking error signal may not be properly formed in the ROM area.
Also, since the offset and the decrease of amplitude of the tracking error signal substantially occur in the ROM area as compared with the recording area, an offset in the tracking control and a decrease in loop gain take place. Therefore, in the final stage of the track access when the light beam reaches the target track and actuates the tracking control, the pull in of the tracking control into the target track has often failed.